Current limiting circuits are often employed when driving a load, to prevent component damage in the event of an overload, such as short circuit. One common technique is to limit the current to some maximum value. The major problem with this method is that under short circuit conditions, the controlling element is subjected to the full limiting current and the full input voltage. This can result in very high power dissipation, which can destroy the limiter.
Another method is fold back current limiting. This technique uses a design which passes circuit up to some "trip" level, and above that level starts to cut back on the output voltage. The design is such that as the output voltage cuts back, the trip current is lowered. This positive feedback condition results in little or no output voltage from the current limiter and little or no current flow. After the overload is removed, the circuitry should recover and provide full output current once again, up to the trip level. A problem with this technique is unreliable startup into loads which draw a surge current, such as capacitors and incandescent lamps.
Both of the methods discussed also lack a means of indicating that an intermittent overload has occurred.
What is needed is a circuit which will efficiently provide output current up to a preset level and, in the event of an overload, shutoff completely and remain in the off state until reset. When in the off state, no current should flow and no power should be dissipated.